The present invention relates to a circuit for limiting the regenerative fault currents developed during inversion operation of a drive system for a direct current motor.
It is common practice to operate direct current motors in a forward motoring mode of operation and a reverse inversion mode of operation. For example, in operating a mine hoist, motoring current is supplied to the motor to lift a payload up from the mine while the motor is operated in a reverse inversion mode to lower a payload into the mine.
During operation of the drive system in the inversion mode, the motor acts like a generator to put energy back into the power supply which is an AC line voltage connected through an AC to DC converter across the armature terminals of the motor. When operation in the inversion mode, the drive system is prone to regenerative currents or inversion fault currents which have been known to damage various components used in the drive system including the mechanical drive train. These inversion fault currents can be caused by a number of abnormal conditions such as, for example, a dip in the AC line voltage, a commutation failure in the converter driving the motor, or a short circuit condition associated with the motor.
In order to safeguard against inversion fault currents, transformers connected between the AC line and the power converter have been designed with a voltage margin above that of the operating voltage of the motor. This however adversely affects the power factor and increases the VAR demand from the AC line.
It is also known to discharge the inversion or regenerative currents by connecting a discharge resistor at all times across the output of the supply. However, this practice results in wasted power. Further, direct current contactors and interlock networks to connect the discharge resistor across the load when required have been used to protect the motor and circuitry. This solution has proven costly due to the cost of the high speed breaker used. In Canadian Pat. No. 830,070 issued Dec. 16, 1969 to Krajewski there is disclosed an energy dissipating circuit that connects a discharge resistor across the output of the supply when the regenerative current exceeds a predetermined amount. The resistor is switched into circuit by the use of a thyristor whose gate electrode senses a voltage rise above an acceptable level.